Wireless power pioneered by Tesla a century ago can be classified as radiative and non-radiative. For non-radiative applications, most of the low-power and medium/high power wireless power applications have their power flow guided by coil-resonators. In many low-power applications such as sensors and RFID devices, replacing the batteries has been a maintenance problem in industry. A well-designed omni-directional wireless charging system is therefore a highly attractive and economic option for charging a multiple of devices simultaneously.
So far, the majority of the non-radiative wireless power systems have the power flow either in one direction (i.e., 1-dimensional power flow) or two directions on the same plane (i.e., 2-dimensional power flow). However, three recent reports published by Wang et al. (see, D. Wang, Y. Zhu, Z. Zhu, T. T. Mo and Q. Huang, “Enabling multi-angle wireless power transmission via magnetic resonant coupling”, International Conference on Computing and Convergence Technology (ICCCT) 2012, pp: 1395-1400; hereinafter “Wang”), Jonah et al. (see, O. Jonah, S. V. Georgakopoulos and M. M. Tentzeris, “Orientation insensitive power transfer by magnetic resonance for mobile devices”, IEEE Wireless Power Transfer, Perugia, Italy, 15-16 May 2013, pp: 5-8; hereinafter “Jonah”) and Kathleen O'Brien (Ph.D Thesis: “Inductively coupled radio frequency power transmission system for wireless systems and devices”, Technische Universitat Dresden, 5 Dec. 2005; hereinafter “O'Brien”) explore the possibility of omni-directional wireless power. O'Brien describes a transmitter system comprising three orthogonal coils and a receiver system also comprising three orthogonal coils. However, in modern applications such as the receiver coils for mobile phones and radio-frequency identity (RFID) tags, the format factor of the mobile devices requires the receiver coil to be a planar one. So a receiver system with three orthogonal coils is not suitable. Wang utilizes orthogonal coils to reduce the effect of small mutual inductance when the receiver coil is perpendicular to one of the transmitter coils, in which two separate orthogonal coils are driven by a single power source with the same ac current, that is, the two separate coils are connected in series. This is why the receiver coil can pick up maximum power at an angle of 45° between the two orthogonal transmitter coils, where the vectorial sum of the two co-axial magnetic field vectors from the two orthogonal coils is maximum if the two coil currents are identical. Wang also suggests the extension to the 3-D structure based on 3 separate orthogonal coils that are connected in series and fed by the same current. In fact, Wang considers the open ended coils as antennas, and use the parasitic coil inductance and capacitance to form an equivalent LC circuit. However, considering the coils are considered as antennas, such a design approach based on impedance matching or the maximum power transfer theorem would have the following limitations:
1) The length of the wire used to implement the resonant circuit is comparable to the wavelength at the resonance frequency. Both of the transmitter and receiver coils are one quarter of the wavelength at the resonant frequency. This approach is therefore dimension-dependent and is restrictive in terms of the relative sizes of transmitter and receiver coils.
2) Due to the usually low parasitic capacitance in open-ended coil, the resonant frequency and therefore the operating frequency is usually high. High-frequency ac power sources are usually more expensive than low-frequency ac power sources.
Jonah discloses that a 3-coil receiver structure with 3 orthogonal open-ended coils is placed inside a similar but larger 3-coil transmitter structure also with open-ended coils. The 3 orthogonal transmitter coils are connected in series and driven with the same ac current. It was demonstrated that wireless power transfer to the 3-coil receiver unit can be achieved regardless of the orientation of the receiver unit inside the transmitter structure. However, this orientation-insensitive feature is only possible if the receiver has 3 orthogonal coils. For RFID tags applications, it is more likely to have a single planar coil in the RFID tag as a receiver coil. So the approach proposed by Jonah is not suitable for a single-coil receiver.
Furthermore, both Wang and Jonah adopt the impedance matching based on the maximum power transfer (MPT) method, which results that the system energy efficiency will not exceed 50%. The use of the same current in the orthogonal coils (i.e. identical current control) also does not generate a magnetic field vector that points in all directions in a 3-dimensional (3-D) manner—which is an essential feature for true omni-directional wireless power transfer.
W. M. Ng and the inventors of this invention have previously proposed the non-identical current control method that can generate rotating magnetic field in 2-dimensional and 3-dimensional space for omni-directional wireless charging systems; see, U.S. patent application Ser. No. 13/975,409, entitled “Wireless Energy Transfer Systems” and filed on Aug. 26, 2013, which is incorporated herein by reference in its entirety (hereinafter “WMNG1”).
WMNG1 provides a non-identical current control method for omni-directional wireless power system using three orthogonal closed loop coils. FIG. 1 shows a typical winding structure of a 3-dimensional (3D) omni-directional transmitter comprising 3 orthogonal coils in the x-, y- and z-plane. In practice, each coil is connected to a series capacitor to form a coil-resonator. Each resonator is driven by an AC power source. For genuine omni-directional wireless power transfer, it is necessary for the orthogonal coil current to be non-identical with each other. The 3 coil currents, I1, I2, I3, can generally be expressed respectively as:I1=Im1 sin(ωt)  (1)I2=Im2 sin(ωt+α)  (2)I3=Im3 sin(ωt+β)  (3);where ω is the angular frequency of the currents, t is the time variable, Imx is the current magnitude of coil-x (for x=1, 2, 3); α and β are two angular displacements. To achieve omni-directional wireless power transmission, rotating magnetic field vectors can be generated by either (i) current amplitude modulation, (ii) phase angle control or (iii) frequency modulation described by WMNG1.
For example, the amplitude modulation approach is illustrated with the following example. Let:
                    I                  m          ⁢                                          ⁢          1                    =              I        m              ,                  ⁢                  I                  m          ⁢                                          ⁢          2                    =                        I          m                ⁢        sin        ⁢                                  ⁢                  (                                    ω              2                        ⁢            t                    )                      ,                  ⁢                  I                  m          ⁢                                          ⁢          3                    =                        I          m                ⁢        sin        ⁢                                  ⁢                  (                                                    ω                2                            ⁢              t                        +                          π              2                                )                      ,                  ⁢          α      =                        π          2                ⁢                                  ⁢        and                        β      =              π        2              ,  where ω2 is another angular frequency different from ω. Equations (1)-(3) become:
                              I          1                =                              I            m                    ⁢          sin          ⁢                                          ⁢                      (                          ω              ⁢                                                          ⁢              t                        )                                              (        4        )                                          I          2                =                              [                                          I                m                            ⁢                              sin                ⁡                                  (                                                            ω                      2                                        ⁢                    t                                    )                                                      ]                    ⁢          sin          ⁢                                          ⁢                      (                                          ω                ⁢                                                                  ⁢                t                            +                              π                2                                      )                                              (        5        )                                          I          3                =                              [                                          I                m                            ⁢                              sin                ⁡                                  (                                                                                    ω                        2                                            ⁢                      t                                        +                                          π                      2                                                        )                                                      ]                    ⁢                                          ⁢                      sin            ⁡                          (                              ω                ⁢                                                                  +                                  π                  2                                            )                                                          (        6        )            
Based on this amplitude modulation approach, the trajectory of the magnetic field vector will form a 3-dimensional sphere as shown in FIG. 2, which is a confirmation of the true omni-directional wireless power system. According to the method proposed by WMNG1, a 2-dimensional prototype based on a 2-orthogonal-coil transmitter system has been successfully demonstrated in FIG. 3 by W. M. Ng et al. (see, W. M. Ng, C. Zhang, D. Lin, and S. Y. R. Hui, “Two- and three-dimensional omni-directional wireless power transfer,” IEEE Transactions on Power Electronics Letters, in press, hereinafter “WMNG2”). WMNG2 has shown that under the identical current method, the energy transfer efficiency is close to zero in some angles as shown in FIG. 4, implying that the identical current control in the 2 orthogonal coils will not generate true omni-directional wireless power in the 2-dimensional plane. But the non-identical current control proposed by WMNG1 and WMNG2 can generate fairly evenly distributed energy efficiency over 360° as shown in FIG. 5, confirming the true omni-directional nature of the wireless power transfer systems in this 2-dimensional prototype.
However, the results of FIG. 4 and FIG. 5 obtained by the inventors (i.e., W. M. Ng et al.) led us to consider a new problem. By comparing the results in FIG. 4 and FIG. 5, it shows that if the magnetic flux direction is controlled in a specific way, it is possible to achieve a high energy efficiency of 73% at certain angular positions as shown in FIG. 4. If we let the magnetic field vector to point in all directions over the spherical surface shown in FIG. 2, the average energy efficiency over all the angles is high, but the maximum energy efficiency is only about 60% as shown in FIG. 5, which is less than 73% as shown in FIG. 4.
Reference to any prior art in the description is not, and should not be taken as an acknowledgement or any form of suggestion that this prior art forms part of the common general knowledge or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.